The aim of this research program is the establishment of a rigorous and comprehensive theory of developmental learning and cognition. The theory aimed at involves the problem solving process. It involves the search for (and acquisition of) the behaviors demanded by the situation. The basic theoretical conception is that the child learns in the discrimination learning situation by testing hypotheses (Hs) or response rules. These Hs are assumed tested according to some more-or-less efficient plan or system, here called an hypothesis sampling system, abb. Sy. Two distinct varieties of Sys have been identified: (a) Strategies, that involve logically specifiable sequences of Hs which in principle ultimately lead to problem solution, and (b) stereotypes, which involve S's continued use of the same H despite disconfirmation. Three of each type of Sy have been delineated and observed with some frequency in children previously studied. Since the theoretical conception underlying H testing and the generating of Sys is not well- understood at present, my research objective is to elucidate this conception. Research and theoretical problems may be considered under the following general headings: (a) Processes underlying H testing: What cognitive processes such as attention, verbal coding and recoding, imagery, memory storage and retrieval, rehearsal, and logical inference underlie H testing in the discrimination learning task? (b) Hypothesis sampling systems: What Sys are available to and used by children of different developmental levels? Under what conditions are more and less sophisticated (in terms of information processing efficiency) Sys observed? What is the relationship between observed Sys and the stage- dependent cognitive structures assumed to underlie preoperational, concrete operational, and formal operational behaviors in Piagetian theory? (c) Empirical problems: These are numerous. The chief topics investigated are verbal mediational processes, attentional factors, modeling and rule provision, and stimulus factors.